Refactored SocketCAN select() call; fixes #83

Features:
- The driver will throw if it's fed a non-existing or malfunctioning interface during initialization
- When an interface becomes down/disconnected while the node is running, the driver will silently
  exclude it from the IO loop and continue to run on the remaining interfaces.
- When all interfaces become down/disconnected, the driver will throw AllIfacesDownException() from
  SocketCanDriver::select().

libuavcan_drivers/linux/include/uavcan_linux/exception.hpp

@@ -29,4 +29,13 @@ public:
     int getErrno() const { return errno_; }
 };
 
+/**
+ * This exception is thrown when all available interfaces become down.
+ */
+class AllIfacesDownException : public Exception
+{
+public:
+    AllIfacesDownException() : Exception("All ifaces are down") { }
+};
+
 }